Long-Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length TSUBFRAME=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RBs), where a RB corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent RBs in the time direction (1.0 ms) is known as a RB pair. RBs are numbered in the frequency domain starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. In particular, in each subframe, the base station transmits control information about the terminals (i.e., User Equipment devices (UEs)) to which data is transmitted in the current downlink subframe. This control signaling, which is carried over the Physical Downlink Control Channel (PDCCH), is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe, where the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Release 11 onwards, the above-described resource assignments can also be scheduled on the Enhanced Physical Downlink Control Channel (EPDCCH). For Release 8 to Release 10 only the PDCCH is available.
The reference symbols shown in FIG. 3 are the Cell-Specific Reference Symbols (CRSs). The CRSs are used to support multiple functions including fine time- and frequency-synchronization and channel estimation for certain transmission modes.
In a cellular communications system, there is a need to measure the channel conditions in order to know what transmission parameters to use. These parameters include, e.g., modulation type, coding rate, transmission rank, and frequency allocation. This applies to Uplink (UL) as well as Downlink (DL) transmissions.
The scheduler that makes the decisions on the transmission parameters is typically located in the base station (i.e., the enhanced or evolved Node B (eNB)). Hence, the scheduler can measure channel properties of the UL directly using known reference signals that the terminals (i.e., UEs) transmit. These measurements then form a basis for the UL scheduling decisions that the eNB makes, which are then sent to the UEs via a DL control channel. Conversely, for the DL, the scheduler receives Channel State Information (CSI) feedback from the terminals, which is taken into consideration by the scheduler when selecting the transmission parameters for the DL transmissions to those terminals.
In LTE Release 8, CRSs are used in the DL for CSI estimation and feedback, and for channel estimation for demodulation. CRSs are transmitted in every subframe and are defined to support up to four Antenna Ports (APs). In LTE Release 10, to support up to eight APs, CSI Reference Signals (CSI-RSs) are defined for the UE to measure and feed back CSI relating to the multiple APs. Each CSI-RS resource consists of two Resource Elements (REs) over two consecutive OFDM symbols, and two different CSI-RSs (for two different APs) can share the same CSI-RS resource (two REs) by Code-Division Multiplexing (CDM). Also, a CSI-RS can be transmitted once per 5, 10, 20, 40, or 80 ms, where this timing is referred to as the CSI-RS periodicity. Therefore. CSI-RS has lower overhead and lower duty-cycle as compared to CRS. On the other hand, unlike CRS, CSI-RS is not used as a demodulation reference. Different CSI-RSs can also be transmitted with different offsets in the subframe, where the offset of the CSI-RS within the subframe is referred to as the CSI-RS subframe offset. When a CSI-RS is configured, the UE measures the channel for a given AP at each time instant and may interpolate the channel in between CSI-RS occasions to estimate the dynamically varying channel, e.g., by one interpolated sample per 1 ms instead of, e.g., one measured sample each 5 ms.
FIGS. 4A and 4B show examples of mappings from different CSI-RS configurations to REs in a RB pair. FIG. 4A illustrates the mapping for one or two APs, where 20 configurations are possible. The two CSI-RSs of the two APs of a particular cell can be transmitted by, for instance, configuration 0 by CDM, while CSI-RSs of APs of other neighboring cells can be transmitted by configuration j, with 1<=j<=19, to avoid reference signal collisions with the CSI-RS in the cell. FIG. 4B shows the mapping for four APs, where 10 configurations are possible. The four CSI-RSs of the four APs of a particular cell can be transmitted by, for instance, configuration 0 by CDM, while CSI-RSs of APs of other neighboring cells can be transmitted by configuration j, with 1<=j<=9.
The OFDM symbols used by the two consecutive REs for one CSI-RS are Quadrature Phase Shift Keying (QPSK) symbols, which are derived from a specified pseudo-random sequence. To randomize the interference, the initial state of the pseudo-random sequence generator is determined by the detected cell Identifier (ID) or a virtual cell ID configured to the UE by Radio Resource Control (RRC) signaling. CSI-RS with such non-zero-power OFDM symbols are called Non-Zero-Power (NZP) CSI-RS.
On the other hand, Zero-Power (ZP) CSI-RS can also be RRC-configured to the UE for the purpose of Interference Measurement (IM) (in Transmission Mode 10 (TM10) only), or for the purpose of improving the CSI estimation in other cells (in Transmission Mode 9 (TM9) or TM10). However, the CSI-RS mapping with four APs will always be used by the ZP CSI-RS. For example, in FIG. 4B, if configuration 0 with NZP CSI-RS is used by cell A to estimate the CSI of the two APs in cell A, configuration 0 with ZP CSI-RS (a total of four REs per RB pair) can be used by the neighboring cell B to minimize the DL interference to cell A over the four REs in configuration 0, such that the CSI estimation of the two APs in cell A can be improved.
In LTE TM10, up to four CSI processes and three NZP CSI-RS can be configured for a UE by RRC signaling. These four CSI processes can, for instance, be used to acquire CSI for APs in up to three different cells (or Transmission Points (TPs) within the same cell) in a Coordinated Multipoint (CoMP) framework. The four CSI processes can also be assigned to multiple different beams transmitted from the same eNB using an array antenna that is capable of beamforming in azimuth, elevation, or both (i.e., Two-Dimensional (2D) beamforming). See 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 36.213 V12.3.0, 3GPP TS 36.331 V12.3.0, and 3GPP TS 36.211 V12.3.0 for complete LTE specifications on how CSI processes and CSI-RS configurations are set up. A beam of a transmitted signal, such as a CSI-RS, is obtained by transmitting the same signal from multiple antenna elements in an array, but with individually controlled phase shifts (and potentially amplitude tapering) for each antenna element. The resulting radiation pattern of the transmitted signal thus has a different beam width and main pointing direction compared to the antenna element radiation pattern. Hence, a beamformed signal, such as beamformed CSI-RS, is obtained. Typically, the antenna elements at the transmitter are closely spaced, as to achieve correlated channels, which makes the beamforming more effective. The benefits of beamforming is reduced interference (due to the typically narrow beam width of the transmitted signal) and increased effective channel gain (due to the applied beamforming phase shifts at the transmitter which ensure a coherent addition of the signals from each transmit antenna at the receiver).
In order for the UE to derive the correct CSI, each CSI process in TM10 is associated (and configured by RRC signaling) with a signal hypothesis and an interference hypothesis. The signal hypothesis describes which NZP CSI-RS reflects the desired signal. The interference is measured in a configured CSI-IM resource, which is similar to a CSI-RS with four REs per Physical Resource Block (PRB) pair, which the UE uses for interference measurements. To better support the IM in CoMP, CSI-IM is standardized and is based on the ZP CSI-RS. Therefore, each of the up to four CSI processes consists of one NZP CSI-RS and one CSI-IM.
For a TM9 UE, only a single CSI process can be configured, and no CSI-IM is defined. The IM is thus unspecified in TM9. There is however still a possibility to get CSI feedback from two different Subframe (SF) sets: SF set 1 and SF set 2. For instance, based on, e.g., the Almost Blank Subframe (ABS) information signaled over X2, a pico eNB can configure a UE to feed back CSI for both protected (i.e., Reduced Power Subframes (RPSF)) subframes (where a corresponding macro eNB has reduced activity) and CSI for unprotected subframes in two different CSI reports. This gives the pico eNB information to perform link adaptation in the two types of subframes differently, depending on whether it is a protected subframe or not. It is also possible for a UE configured in TM10 to use both subframe sets and multiple CSI processes.
In LTE, the format of the CSI reports are specified in detail and may contain Channel Quality Information (CQI), Rank Indicator (RI), and Precoding Matrix Indicator (PMI). See 3GPP TS 36.213 V12.3.0. The reports can be wideband or applicable to subbands. They can be configured by a RRC message to be sent periodically or in an aperiodic manner or triggered by a control message from the eNB to a UE. The quality and reliability of the CSI are crucial for the eNB in order to make the best possible scheduling decisions for the upcoming DL transmissions.
The LTE standard does not specify how the UE should obtain and average the CSI-RS and CSI-IM measurements from multiple time instants, i.e., subframes. For example, the UE may measure over a time frame of multiple subframes, unknown to the eNB and combine several measurements in a UE-proprietary way to create the CSI values that are reported, either periodically or triggered.
In the context of LTE, the resources (i.e., the REs) available for transmission of CSI-RS are referred to as “CSI-RS resources,” In addition, there are also “CSI-IM resources.” The latter are defined from the same set of possible physical locations in the time/frequency grid as the CSI-RS, but with zero power, hence ZP CSI-RS. In other words, they are “silent” CSI-RSs and when the eNB is transmitting the shared data channel, it avoids mapping data to those REs used for CSI-IM. These are intended to give a UE the possibility to measure the power of any interference from another transmitter other than the serving node of the UE.
Each UE can be configured with one, three, or four different CSI processes. Each CSI process is associated with one CSI-RS and one CSI-IM resource where these CSI-RS resources have been configured to the UE by RRC signaling and are thus periodically transmitted/occurring with a periodicity of T and with a given subframe offset relative to the frame start.
If only one CSI process is used, then it is common to let the CSI-IM reflect the interference from all other eNBs, i.e., the serving cell uses a ZP CSI-RS that overlaps with the CSI-IM, but in other adjacent eNBs there is no ZP CSI-RS on these resources. In this way the UE will measure the interference from adjacent cells using the CSI-IM.
If additional CSI processes are configured to the UE, than there is possibility for the network to also configure a ZP CSI-RS resource in the adjacent eNB that overlaps with a CSI-IM resource for this CSI process for the UE in the serving eNB. In this way the UE will feed back accurate CSI also for the case when this adjacent cell is not transmitting. Hence, coordinated scheduling between eNBs is enabled with the use of multiple CSI processes and one CSI process feeds back CSI for the full interference case and the other CSI process feeds back CSI for the case when a (strongly interfering) adjacent cell is muted. As mentioned above, up to four CSI processes can be configured to the UE, thereby enabling feedback of four different transmission hypotheses.
The PDCCH/EPDCCH is used to carry Downlink Control Information (DCI) such as scheduling decisions and power control commands. More specifically, the DCI includes:                DL scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid Automatic Repeat Request (ARQ) information, and control information related to spatial multiplexing (if applicable). A DL scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid ARQ acknowledgements in response to DL scheduling assignments.        UL scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid ARQ related information. A UL scheduling grant also includes a command for power control of the PUSCH.        Power control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
The PDCCH/EPDCCH region carries one or more DCI messages, each with one of the formats above. As multiple terminals can be scheduled simultaneously, on both DL and UL, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH physical resources. Furthermore, to support different radio channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio channel conditions.
Against this backdrop, future cellular communications networks are expected to utilize beamforming where the number of beams may exceed the number of CSI-RS resources. In addition, existing and future cellular communications networks sometimes use a multi-layer radio access network including a number of coverage cells (e.g., macro cells controlled by eNBs) and a number of capacity cells (e.g., pico cells controlled by pico eNBs). As such, there is a need for systems and methods that enable improved CSI-RS configuration, particularly for cellular communications networks that utilize beamforming and/or multi-layer radio access networks.